Get them soda blasted 
Yeah good idea I can soda blast them for you. Sodium Bicarbonate works great on baked on grime and grease. Pistons and other internal engine parts look like new when finished
I use children’s tears and a scrub pad. Works everytime.
green nitro. or parts cleaner i used to clean tranny parts for my boss when he rebuilt them had 2 different parts cleaners
Oven cleaner